This invention relates in general to composite components, and in particular to methodology for fabricating a discontinuous-fiber, ceramic matrix composite component wherein a mixture of discontinuous fibers and an excess of ceramic precursor resin is introduced under vacuum to a mold cavity whereby the excess resin provides a fiber transport medium which, as the fibers compact at a vacuum aperture of the mold cavity and there function as a filter, is permitted to escape through the aperture while the fibers so transported are retained within the cavity for structural incorporation.
In general, the use of a mold cavity to manufacture various components is widely practiced, with among the most frequent usage being the construction of plastic products functional for immediate use or for incorporation into a secondary product. Simultaneously, because of superior high-temperature strength characteristics, the use of discontinuous-fiber ceramic matrix composite materials for the construction of various components is well recognized. However, employment of a traditional compression molding process for such composite materials, especially in the fabrication of a relatively long and thin-walled ceramic matrix component, many times creates a twofold problem. First, in order to mix a resin/fiber mixture to a flowable consistency for mold introduction, a finished product generally results that have resin-rich areas toward the bottom of the mold cavity and resin-starved areas toward the top (compression plunger side) as the resistance to flow of the molding cavity increases as the mold fills. As this occurs, the compression plunger squeezes resin out of the fiber at the top of the mold in a manner much like that of squeezing water from a sponge. Second, as the conventional compression plunger of the mold tool pushes the resin/fiber mixture into the mold cavity, moisture-starved fibers, especially near the plunger entry, are produced within the mixture. Consequently, sub-standard end products can be produced.
In view of the potential shortfalls that can occur in traditional mold-forming techniques, it is apparent that a need is present for mold methodology wherein adequate and uniform distribution of quality fibers occurs throughout a ceramic matrix composite component to be formed within a mold cavity. Accordingly, a primary object of the present invention is to provide methodology for fabricating a discontinuous-fiber, green-state ceramic matrix composite component wherein fiber concentration, fiber quality and fiber distribution are in accord with end-product structural requirements.
Another object of the present invention is to provide such methodology wherein a resin-rich resin/fiber mixture is introduced into a mold cavity, preferably via a charging station upstream of the mold cavity, from which a vacuum aperture permits movement of the fiber throughout the mold cavity followed by extraction of excess resin and retention of fibers.
Still another object of the present invention is to provide such methodology wherein vacuum-driven resin/fiber mixture entry into the mold cavity eliminates active plunger compression activity and attendant resin starvation and potential fiber damage.
These and other objects of the present invention will become apparent throughout the description thereof which now follows.
The present invention is a method of fabricating a discontinuous-fiber, green-state ceramic matrix composite component comprising three steps in sequence. The initial step is the preparation of a mixture of, first, discontinuous fibers in a quantity equal to about 100% of a desired end-product fiber quantity thereof, and, second, a pre-ceramic resin in an excess quantity greater than about 150% of a desired end-product resin quantity thereof. The discontinuous fibers are preferably cured-resin coated and preferably prepared in accord with the methodology taught in commonly assigned pending U.S. patent application Ser. No. 09/170,004, filed Oct. 13, 1998, now U.S. Pat. No. 6,066,004 and incorporated herein in its entirety. Briefly, discontinuous fiber preparation is achieved by first thoroughly wetting or coating a fiber with a resin, second, permitting the fiber to drip dry to remove excess resin, third, chopping the fiber into a discontinuous state, and fourth, curing the resin of these discontinuous fibers.
The mixture of discontinuous fibers and ceramic precursor resin so prepared then is poured into the charge station forward of the opening into a cavity of a molding tool and a vacuum is applied to the cavity through a vacuum aperture leading from the cavity. This step draws the mixture toward the vacuum aperture and consequently compacts a quantity of fibers within the cavity at the aperture site such that the fibers function as a filter to efficiently retain within the cavity all fibers within the original mixture while removing under vacuum the excess resin that provided an effective vehicle for carrying and dispersing the discontinuous fibers. As the final step, the molding tool is heated to a temperature and for a time sufficient to cure the resin/fiber mixture within the cavity to thereby fabricate the composite component in a structurally sound manner.
In presently preferred methodology, the resin is a pre-ceramic polymer and is initially present in an amount of about 200% of the desired end-product resin quantity and with a viscosity between 5,000 and 10,000 CPS. Polymer-derived ceramic precursor resins are non-limitedly exemplified in BLACKGLAS (AlliedSignal Corporation [e.g. resin no. 493E]), CERASET (AlliedSignal Corporation), SYLRAMIC (Engineering Ceramics, Inc.), and STARFIRE (Starfire Corporation) resins, while fiber identity is non-limitedly exemplified in a group consisting of alumina, Nextel 312, Nextel 440, Nextel 510, Nextel 550, silicon nitride, silicon carbide, HPZ, graphite, carbon, peat, and mixtures thereof. The fibers can be coated with an interface material to inhibit the resin from adhering directly to the fibers. Such interface material non-limitedly can be chosen from the group consisting of carbon, silicon nitride, silicon carbide, silicon carboxide, and boron nitride, and can be of a thickness of about 0.1 to about 5.0 microns. Preferred fiber configuration is a cylindrical shape having a length between about 0.125 inch to about 0.25 inch, thus, for purposes of size comparison, very much like the appearance of rice.
Pouring the resin/fiber mixture into the charge station situated upstream from and in communication with the mold cavity and applying vacuum to the cavity through a vacuum aperture at the distal end of the cavity draws resin and fiber toward the vacuum aperture while causing movement (due to pressure differential) of a standard compression plunger into the charge station to thereby disperse an effective resin/fiber mixture within the mold subsequent to vacuum removal of excess resin originally present as a vehicle for fiber transport. Thereafter, the resin/fiber mixture within the mold is conventionally cured within the mold tool by heating the tool to the appropriate temperature for the appropriate time in accord with curing characteristics of the resin. Upon completion of the curing process, the green-state component can be removed from the mold and is ready for pyrolysis to thereby accomplish conversion to a ceramic.